The invention relates to a method for changing the speed of a motor group, in particular for starting or stopping it, the motor group comprising a plurality of squirrel-cage induction motors or synchronous motors and network converters arranged for their control, when the nominal supply power, the acceleration power at the final speed of the motors and the losses of the used power are known.
In several fields of industry it is necessary to start a plurality of electric motors simultaneously. For instance, in paper or metal industry, a plurality of motors are connected to form groups, which should be controlled in a uniform manner, because the groups are mechanically interconnected. For instance, one such group is a drying section of a paper machine, in which a plurality of cylinders driven by motors are mechanically interconnected.
A torque required for accelerating heavy groups of this kind is considerably higher than the torque required for normal production run. Traditionally, heavy groups are started one group at a time and the starting of one group takes about 5 minutes, depending on the final speed. In general, the starting is implemented by controlling the motors with a linear acceleration ramp using a constant torque.
In a large group-drive installation, such as in connection with a paper machine, power supply to the motors is divided into sections that are called supply units or supply groups. One supply unit supplies electric power to a plurality of different electric drives, i.e. through a motor control device to the motors. The supply unit includes various electric devices, such as transformers, bridge coils or high-voltage cables, feed converters and protective devices.
The supply group is electrically rated on the basis of the continuous drive load, which is determined on the basis of the sum of the products of the mechanical torques and speeds required in the production, by using typical coefficients determined for the drive groups in the adding operation.
In connection with motor groups, a conventional drive solution consists of a phase-angle-controlled DC drive that always takes the same nominal power from the network with constant torque, the nominal power being the vector sum of the effective power and the idle power. This means that the supply devices have to be rated on the basis of the nominal power in connection with the DC drive.
A more up-to-date drive solution is an AC drive, whose input power consists, in practice, of only effective power, making a better use of supply group components. The torque being constant, the effective power increases linearly as the speed increases. The AC drive has a separate network converter, which is used for electronic power rectification for all separate converters controlling the motors. Because in connection with AC drives there is practically no idle power, large installations, such as the paper machine, need at least one supply group less than the DC drives.
In connection with current paper machines, drying section felt draws are arranged such that several drying sections have to be started at the same time. Thus, it is not possible to use the conventional starting method based on linear acceleration, because at the final stage of the start-up the linear ramp requires considerably higher power than the design power, which results in overloading in front devices, such as supply transformer or supply converter, of the drying section frequency converters. This can be avoided by considerably overrating the front devices of the supply such that a sufficient number of supply transformers and supply units are selected to make sure that their actual power is sufficient to supply all the groups to be started simultaneously also at the final part of the acceleration ramp. However, the overrating increases the costs of devices that are only needed momentarily in the start-up. In addition, the overrating requires considerably more space for the devices.